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(New page: {{BioPsy}} :''"Bilateral symmetry" redirects here. For bilateral symmetry in mathematics, see reflection symmetry.'' [[Image:Commonbuckeye.JPG|thumb|The elaborate patterns on the wing...)
 
 
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{{BioPsy}}
 
{{BioPsy}}
:''"Bilateral symmetry" redirects here. For bilateral symmetry in mathematics, see [[reflection symmetry]].''
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:''"Bilateral symmetry" redirects here.
   
 
[[Image:Commonbuckeye.JPG|thumb|The elaborate patterns on the wings of [[butterfly|butterflies]] are one example of biological symmetry.]]
 
[[Image:Commonbuckeye.JPG|thumb|The elaborate patterns on the wings of [[butterfly|butterflies]] are one example of biological symmetry.]]
   
'''Symmetry in biology''' is the balanced distribution of duplicate body parts or shapes. The body plans of most [[multicellular organism]]s exhibit some form of [[symmetry]], either '''radial symmetry''' or '''bilateral symmetry'''. A small minority exhibit no symmetry (are asymmetric).
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'''Symmetry in biology''' is the balanced distribution of duplicate body parts or shapes. The body plans of most multicellular organisms exhibit some form of [[symmetry]], either '''radial symmetry''' or '''bilateral symmetry'''. A small minority exhibit no symmetry (are asymmetric).
   
 
In [[nature]] and [[biology]], symmetry is approximate. For example, plant leaves, while considered symmetric, will rarely match up exactly when folded in half.
 
In [[nature]] and [[biology]], symmetry is approximate. For example, plant leaves, while considered symmetric, will rarely match up exactly when folded in half.
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==Radial symmetry==
 
==Radial symmetry==
 
[[Image:Haeckel Actiniae.jpg|thumb|right|These sea anemones display radial symmetry. ([[Photographic plate|Plate]] from [[Ernst Haeckel]]'s ''[[Kunstformen der Natur]]'').]]
 
[[Image:Haeckel Actiniae.jpg|thumb|right|These sea anemones display radial symmetry. ([[Photographic plate|Plate]] from [[Ernst Haeckel]]'s ''[[Kunstformen der Natur]]'').]]
These organisms resemble a pie where several cutting [[plane (mathematics)|plane]]s produce roughly identical pieces. An organism with radial symmetry exhibits no left or right sides. They have a top and a bottom ([[Anatomical terms of location|dorsal and ventral]] surface) only.
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These organisms resemble a pie where several cutting planes produce roughly identical pieces. An organism with radial symmetry exhibits no left or right sides. They have a top and a bottom ([[Anatomical terms of location|dorsal and ventral]] surface) only.
 
====Animals====
 
====Animals====
 
Most radially symmetric animals are symmetrical about an axis extending from the center of the oral surface, which contains the mouth, to the center of the opposite, or aboral, end. This type of symmetry is especially suitable for [[sessile]] animals such as the [[sea anemone]], floating animals such as [[jellyfish]], and slow moving organisms such as [[starfish]] (see special forms of radial symmetry). Animals in the phyla [[cnidaria]] and [[echinoderm|echinodermata]] exhibit radial symmetry.
 
Most radially symmetric animals are symmetrical about an axis extending from the center of the oral surface, which contains the mouth, to the center of the opposite, or aboral, end. This type of symmetry is especially suitable for [[sessile]] animals such as the [[sea anemone]], floating animals such as [[jellyfish]], and slow moving organisms such as [[starfish]] (see special forms of radial symmetry). Animals in the phyla [[cnidaria]] and [[echinoderm|echinodermata]] exhibit radial symmetry.
 
Cnidarians are one of the simplest animal forms on this planet. Radial also means it only has one line of symmetry.
 
Cnidarians are one of the simplest animal forms on this planet. Radial also means it only has one line of symmetry.
   
====Plants====
 
Many [[flower]]s are radially symmetric (also known as [[actinomorphic]]). Roughly identical [[petal]]s, [[sepal]]s, and [[stamen]] occur at regular intervals around the center of the flower.
 
   
 
===Special forms of radial symmetry===
 
===Special forms of radial symmetry===
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Members of the phyla [[echinodermata]] (such as [[sea star|starfish]] and [[sea urchin]]s) have parts arranged around the axis of the mouth in five equal sectors. Being [[bilateria]]n animals however, they initially develop biradially as larvae, then gain pentaradial symmetry later on. The [[radiolarian]]s demonstrate a remarkable array of pentamerism forms. Examples include the Pentaspheridae, the Pentinastrum group of general in the Euchitoniidae, and Cicorrhegma (Circoporidae).
 
Members of the phyla [[echinodermata]] (such as [[sea star|starfish]] and [[sea urchin]]s) have parts arranged around the axis of the mouth in five equal sectors. Being [[bilateria]]n animals however, they initially develop biradially as larvae, then gain pentaradial symmetry later on. The [[radiolarian]]s demonstrate a remarkable array of pentamerism forms. Examples include the Pentaspheridae, the Pentinastrum group of general in the Euchitoniidae, and Cicorrhegma (Circoporidae).
   
*'''Plants'''
 
[[Image:Sterappel_dwarsdrsn.jpg|right|thumb|Apple cut horizontally, showing pentamerism]]
 
[[Flowering plant]]s demonstrate symmetry of five more frequently than any other form.
 
 
Around 1510–1516 A.D., [[Leonardo da Vinci]] determined that in many plants a sixth leaf stands above the first. This arrangement later became known as 2/5 [[phyllotaxis|phyllotaxy]], a system where repetitions of five leaves occur in two turns of the axis. This is the most common of all patterns of leaf arrangement.
 
 
Various fruits also demonstrate pentamerism, a good example of which is seen in the arrangement of the seed [[carpel|carpels]] in an apple.
 
   
 
====Hexamerism and octamerism====
 
====Hexamerism and octamerism====
[[Coral]]s and [[sea anemones]] (class ''[[Anthozoa]]'') are divided into two groups based on their symmetry. The most common corals in the subclass ''[[Zoantharia|Hexacorallia]]'' have a '''hexameric''' body plan; their [[polyp]]s have six-fold internal symmetry and the number of their [[tentacle]]s is a multiple of six.
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Corals and [[sea anemones]] (class ''[[Anthozoa]]'') are divided into two groups based on their symmetry. The most common corals in the subclass ''[[Zoantharia|Hexacorallia]]'' have a '''hexameric''' body plan; their [[polyp]]s have six-fold internal symmetry and the number of their [[tentacle]]s is a multiple of six.
   
 
Corals belonging to the subclass ''[[Alcyonaria|Octocorallia]]'' have polyps with eight tentacles and '''octameric''' radial symmetry.
 
Corals belonging to the subclass ''[[Alcyonaria|Octocorallia]]'' have polyps with eight tentacles and '''octameric''' radial symmetry.
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Bilateral symmetry permits [[streamline|streamlining]], favors the formation of a central nerve center, contributes to [[cephalization]], and promotes actively moving organisms. Bilateral symmetry is an aspect of both chordates and vertebrates.
 
Bilateral symmetry permits [[streamline|streamlining]], favors the formation of a central nerve center, contributes to [[cephalization]], and promotes actively moving organisms. Bilateral symmetry is an aspect of both chordates and vertebrates.
 
===Plants===
 
Flowers such as members of the [[Orchidaceae|orchid]] and [[pea]] families are bilaterally symmetrical (also known as [[zygomorphic]]). The leaves of most plants are also bilaterally symmetrical.
 
   
 
==Asymmetry==
 
==Asymmetry==

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"Bilateral symmetry" redirects here.
File:Commonbuckeye.JPG

Symmetry in biology is the balanced distribution of duplicate body parts or shapes. The body plans of most multicellular organisms exhibit some form of symmetry, either radial symmetry or bilateral symmetry. A small minority exhibit no symmetry (are asymmetric).

In nature and biology, symmetry is approximate. For example, plant leaves, while considered symmetric, will rarely match up exactly when folded in half.

These organisms resemble a pie where several cutting planes produce roughly identical pieces. An organism with radial symmetry exhibits no left or right sides. They have a top and a bottom (dorsal and ventral surface) only.

Radial symmetryEdit

File:Haeckel Actiniae.jpg

These organisms resemble a pie where several cutting planes produce roughly identical pieces. An organism with radial symmetry exhibits no left or right sides. They have a top and a bottom (dorsal and ventral surface) only.

AnimalsEdit

Most radially symmetric animals are symmetrical about an axis extending from the center of the oral surface, which contains the mouth, to the center of the opposite, or aboral, end. This type of symmetry is especially suitable for sessile animals such as the sea anemone, floating animals such as jellyfish, and slow moving organisms such as starfish (see special forms of radial symmetry). Animals in the phyla cnidaria and echinodermata exhibit radial symmetry. Cnidarians are one of the simplest animal forms on this planet. Radial also means it only has one line of symmetry.


Special forms of radial symmetryEdit

TetramerismEdit

Many jellyfish have four radial root canals and thus exhibit tetramerous radial symmetry. This form of radial symmetry means it can be divided into 4 equal parts.

PentamerismEdit

This variant of radial symmetry (also called pentaradial and pentagonal symmetry) arranges roughly equal parts around a central axis at orientations of 72° apart.

  • Animals

Members of the phyla echinodermata (such as starfish and sea urchins) have parts arranged around the axis of the mouth in five equal sectors. Being bilaterian animals however, they initially develop biradially as larvae, then gain pentaradial symmetry later on. The radiolarians demonstrate a remarkable array of pentamerism forms. Examples include the Pentaspheridae, the Pentinastrum group of general in the Euchitoniidae, and Cicorrhegma (Circoporidae).


Hexamerism and octamerismEdit

Corals and sea anemones (class Anthozoa) are divided into two groups based on their symmetry. The most common corals in the subclass Hexacorallia have a hexameric body plan; their polyps have six-fold internal symmetry and the number of their tentacles is a multiple of six.

Corals belonging to the subclass Octocorallia have polyps with eight tentacles and octameric radial symmetry.

Bilateral symmetryEdit

File:Leaf 1 web.jpg

In bilateral symmetry (also called plane symmetry), only one plane, called the sagittal plane, will divide an organism into roughly mirror image halves (with respect to external appearance only, see situs solitus). Thus there is approximate reflection symmetry. Often the two halves can meaningfully be referred to as the right and left halves, e.g. in the case of an animal with a main direction of motion in the plane of symmetry.

AnimalsEdit

Most animals are bilaterally symmetric, including humans (see also facial symmetry), and belong to the group Bilateria. The oldest known bilateral animal is the Vernanimalcula.

Bilateral symmetry permits streamlining, favors the formation of a central nerve center, contributes to cephalization, and promotes actively moving organisms. Bilateral symmetry is an aspect of both chordates and vertebrates.

AsymmetryEdit

The notable exception among animals is the phylum Porifera (sponges) which have no symmetry.

See alsoEdit

ReferencesEdit

  • Fact Monster
  • Heads, Michael. "Principia Botanica: Croizat's Contribution to Botany." Tuatara 27.1 (1984): 26-48.
  • Zoology a website by the Monaco educational service

External linksEdit

{{enWP|Symmetry (biology)}

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